1. Field of the Invention
This invention relates to a pre-insertion resistor mechanism, and, more specifically, to a pre-insertion resistor mechanism for use with circuit interrupting devices. Additionally, the present invention relates to an improved circuit interrupting device into which is incorporated the pre-insertion resistor mechanism of the present invention.
2. Prior Art
The use of a pre-insertion resistor in a circuit interrupting device to protect circuits during closing operations thereof is, in general, well known. See U.S. Pat. Nos. 4,069,406; 4,072,836; and 3,291,947. As the circuit interrupting device is closed, the pre-insertion resistor is connected in parallel with a gap previously opened therein. When the pre-insertion resistor is placed in parallel with the gap, the circuit voltage measured to ground (generally line-to-ground voltage) is dropped thereacross. Accordingly, the current flowing through the resistor is determined by V/Z, where V is the line-to-ground voltage of the circuit and Z is the vector sum of the resistance of the pre-insertion resistor and the surge impedance of inductive and capacitive elements (such as capacitor banks, reactors, bus work) connected to and in the circuit. The current determined by this calculation is often referred to as the inrush current and may achieve a substantially high level for some fraction of a second, especially where the device is used in conjunction with a capacitor bank. Clearly, inrush currents having high magnitudes may damage the circuit. For example, without pre-insertion resistors, the inrush current often reaches values of about 10 to 30 thousand amperes--a magnitude which can lead to distress or damage of the circuit; with a pre-insertion resistor, the initial inrush current is relatively lower-- generally 2-4 thousand amperes--and can be carried by the circuit without undue distress. Following the initial inrush current, the current through the pre-insertion resistor is ultimately limited by the steady state impedance of capacitor banks, and other items (loads, reactors, etc.) connected to and in the circuit. Consequently, following the initial inrush current, the current flow through the pre-insertion resistor is generally within the range of 100 to 400 amperes. When the pre-insertion resistor is first inserted into the circuit in parallel with the gap, its electrode structure must be able to withstand some initial arcing, as is well known. However, in view of the fact that pre-insertion resistors ordinarily carry little or none of the continuous current through the interrupting device, such electrode structure need only be sufficiently robust to withstand initial arcing and momentary currents up to about 4,000 amperes from time to time.
Quickly following the time when the initial inrush current has subsided and the current through the pre-insertion resistor has dropped to the substantially lower level of 100-400 amperes, contacts of the interrupting device re-engage. If the device is switching capacitor banks, the banks discharge directly through the contacts, the current being now limited by the surge impedance of the banks and the bus work. Generally, it has been found that the value of the resistor may be chosen so that--considering the electrical characteristics of the circuit, especially of the banks--the magnitude of the discharge current is substantially the same as the magnitude of the initial inrush current. Thus, if the circuit is able to carry the 2,000-4,000 ampere initial inrush current, it is also able to carry the similar discharge current.
The resistance of the interface between the contacts of circuits interrupting devices is typically on the order of micro-to-milli-ohms, whereas the resistance of pre-insertion resistors is typically on the order of 100 ohms. Accordingly, the engagement of the contacts shunts the majority of the current therethrough so that the pre-insertion resistor is not called upon to thereafter carry any substantial part of the continuous or normal current through the circuit interrupting device.
A wide variety of schemes are known for inserting pre-insertion resistors in parallel with an already open gap between previously separated contacts of a circuit interrupting device, while preventing such pre-insertion resistors from being connected across such gap as it is opened during disengagement of the contacts. Most of these schemes involve complicated linkages, lost motion devices, dashpots and piston-cylinders, which add to the cost of the circuit interrupting device, which are complicated to manufacture and assemble, and which are difficult to adjust.
Accordingly, it is an object of the present invention to provide a mechanism for connecting a pre-insertion resistor in electrical parallel with an open gap between disengaged contacts of a circuit interrupting device, such mechanism being simple, economical, easy to manufacture, and convenient to adjust, or requiring no adjustment.